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State-of-the-art and recent developments in micro/nanoscale pressure sensors for smart wearable devices and health monitoring systems

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Small-sized, low-cost, and high-sensitivity sensors are required for pressure-sensing applications because of their critical role in consumer electronics, automotive applications, and industrial environments. Thus, micro/nanoscale pressure sensors based on micro/nanofabrication and micro/nanoelectromechanical system technologies have emerged as a promising class of pressure sensors on account of their remarkable miniaturization and performance. These sensors have recently been developed to feature multifunctionality and applicability to novel scenarios, such as smart wearable devices and health monitoring systems. In this review, we summarize the major sensing principles used in micro/nanoscale pressure sensors and discuss recent progress in the development of four major categories of these sensors, namely, novel material-based, flexible, implantable, and self-powered pressure sensors. Keywords: M/NEMS, Pressure sensor, Flexible sensor, Piezoresistive sensor, Capacitive sensor, Piezoelectric sensor, Resonant sensor, 2D material
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State-of-the-art and recent developments in micro/nanoscale pressure
sensors for smart wearable devices and health monitoring systems
Ye Chang, Jingjing Zuo, Hainan Zhang, Xuexin Duan
State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072, China
abstractarticle info
Available online 27 December 2019 Small-sized, low-cost, and high-sensitivity sensorsare required for pressure-sensingapplications because of their
critical role in consumer electronics, automotive applications, and industrial environments. Thus, micro/nano-
scale pressure sensors based on micro/nanofabrication and micro/nanoelectromechanical system technologies
have emerged as a promising class of pressure sensors on account of their remarkable miniaturization and per-
formance. These sensors have recently been developed to feature multifunctionality and applicability to novel
scenarios, such as smart wearable devices and health monitoring systems. In this review, we summarize the
major sensing principles used in micro/nanoscale pressure sensors and discuss recent progress in the develop-
ment of four major categories of these sensors, namely, novel material-based, exible, implantable, and self-
powered pressure sensors.
Copyright © 2020 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:
M/NEMS
Pressure se nsor
Flexible sensor
Piezoresistive sensor
Capacitive sensor
Piezoelectric sensor
Resonant sensor
2D material
1. Introduction
A pressure sensor is a transducer that converts an external pressure
stimulus into an electrical or other identiable output signal according
to certain rules.
1
Over the last several decades, the role of pressure sens-
ing in daily life has escalated, leading to the rapid growth of its market
size. According to a recent study, the global market for pressure sensors
is expected to increase to $15.97 billion by the year 2028 from $8.8 bil-
lion in 2018. The major pressure-sensor suppliers in the global market
include Bosch, Denso, Sensata, and Amphenol.
Conventional pressure-sensing devices are mainly based on macro-
scale diaphragm congurations, the deformation of which indicates
the applied pressure. Such sensors provide the advantages of high
stability and large dynamic range, but their bulky size limits their fur-
ther application. Given rapid developments in micro/nanofabrication
and micro/nanoelectromechanical system (M/NEMS) technologies,
micro/nanoscale pressure sensors based on various measurement
principles, e.g., piezoresistive, capacitive, piezoelectric, and resonant
transduction,
26
have received increased research attention. CMOS
compatibility and wafer-scale fabrication have enabled the develop-
ment of a new generation of pressure sensors with high sensitivity,
low cost, and small size to address the needs of current applications.
Thus far, a number of micro/nanoscale pressure sensors have been suc-
cessfully used in consumer electronics devices, automotive applications,
and industrial environments.
7
In addition, some specic sensors have
been demonstrated to be capable of operating in extreme conditions,
such as those applied in the aerospace, marine, and oil industries, with
excellent performance and robustness.
812
Advances in nanomaterials, microelectronics, and exible electron-
ics have allowed the application of micro/nanoscale pressure sensors
to a wider range of scenarios, such as smart wearable devicesand health
monitoring systems.
13,14
In smart wearable devices, a pressure sensor,
especially a pressure sensor matrix, can be used to indicate tactile sig-
nals on human skin; this feature is the main principle behind the so-
called electronic skin(E-skin).
15
The applications of E-skins a re mainly
focused on soft robotics, articial prosthetic replacement, and medical
diagnostics, which present challenges to current micro/nanoscale pres-
sure sensors, such as the entire exibility, easy integration and self-
healing properties of the device. In health monitoring systems, pressure
is a major sign of life because pressure variations in physiology may in-
duce deteriorating actions on body tissues.
16
Therefore, micro/nano-
scale pressure sensors are also increasingly used in mobile biological
monitoring and in vivo pressure measurements.
1719
These sensors
must meet increasing demands, including implantation ability, biocom-
patibility, self-power, and wireless transmission.
Great advances have been achieved in the development of micro/
nanoscale pressure sensors for the past few years. In this review, we
provide a brief introduction of recent progress in micro/nanoscale
pressure sensors applicable to wider usage. First, an overview of
Nanotechnology and Precision Engineering 3 (2020) 4352
Corresponding author.
E-mail address: xduan@tju.ed u.cn (X. Duan).
https://doi.org/10.1016/j.npe.2019.12.006
2589-5540/Copyright © 2020 TianjinUniversity.Publishing Serviceby Elsevier B.V.on behalf of KeAi Communications Co.,Ltd. This is an open access article under the CC BY-NC-ND lic ense
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents lists available at ScienceDirect
Nanotechnology and Precision Engineering
journal homepage: http://www.keaipublishing.com/en/journals/nanotechnology-
and-precision-engineering/
fundamental pressure-sensing principles, including piezoresistivity, ca-
pacitance, piezoelectricity, and resonance, is discussed. Next, we pres-
ent recent advancements in four major categories of micro/nanoscale
pressure sensors, namely, novel material-based, exible, implantable,
and self-powered pressure sensors. Finally, we conclude this review
and outline perspectives on the development of micro/nanoscale pres-
sure sensors.
2. Pressure-sensing principles
2.1. Piezoresistivity
The discovery of the piezoresistive effect can be dated back to 1856
by Lord Kelvin.
20
Several decades afterward, Smith et al.
21
investigated
the piezoresistive effect in semiconductors (e.g., silicon and germa-
nium) and contributed to signicant developments in miniaturized
piezoresistive sensors. Thus far, these sensors have become one of the
most well-known and widely used approaches in sensing applications,
such as force, displacement, ow, and pressure sensing.
22
The basic
principle of a piezoresistive pressure sensor is conversion of the pres-
sure stimulus exerted on the device into a resistance variation that
can be recorded. A piezoresistive pressure sensor typically consists of
a sandwich structure with a piezoresistive material layer intercalated
between a pair of parallel electrodes. The piezoresistive layer should
offer outstanding electrical and mechanical properties and can be de-
signed as beam, cantilever, or diaphragm for specic needs.
Piezoresistive pressure sensors based on this simple structure and
mechanism allow facile fabrication, high sensitivity, short response
times, and easy circuit interfacing. However, the high temperature coef-
cient of piezoresistivity limits the performance of these sensors, which
means these devices require temperature compensation techniques.
1,23
2.2. Capacitance
A typical capacitive pressure sensor converts applied pressure into a
capacitance variation by usinga parallel electrode capacitor. In a typical
conguration, one electrode of the capacitor is deected under pressure
stimuli while the other electrode is xed.The device capacitance follows
the equation C=ε
0
ε
r
A/d, where ε
0
and ε
r
respectively represent the
permittivities of the vacuum and dielectric material between the capac-
itor electrodes and Aand drespectively represent the overlap area and
distance between two electrodes. Deection of the electrode leads to a
change in d(compression force) or A(shear force), resulting in varia-
tions in capacitance that can be measured by a capacitance bridge
circuit.
24
Similar to piezoresistive pressure sensors, capacitive pressure
sensors present the advantages of simple structure, easy fabrication,
high sensitivity, and low cost. In addition, this type of sensor enables
high-temperature adaptability, which satises requirements for appli-
cation to harsh conditions. Nevertheless, nonlinear output signals and
parasitic capacitance remain signicant issues for capacitive pressure
sensors.
2.3. Piezoelectricity
The piezoelectric effect was rst described by the Curie brothers in
1880. When a piezoelectric material is under external stress, its two sur-
faces become positively and negatively charged.
25
This phenomenon
has been used to develop piezoelectric pressure sensors in which pres-
sure stimuli are directly converted into electrical potential variations.
PZT thin lms are conventionally used as active materials, usually
sandwiched between two electrodes, in micro piezoelectric pressure
sensors. ZnO has also been reported as a promising material for piezo-
electric pressure-sensing devices.
26
These miniaturized sensors offer
properties similar to those of sensors based on microfabrication tech-
nology described earlier. Indeed, they are especially suitable for
dynamic pressure-sensing applications because of their impulsive out-
put signals.
14
2.4. Resonance
The current resonant devices are widely used in the sensing eld on
account of their improved sensitivity and reliability. When these devices
are used as pressure sensors, pressure-induced stresses change their
natural frequencies. Compared with conventional pressure sensors, res-
onant pressure sensors have been demonstrated to enable higher sensi-
tivity andprecision becausetheir frequencysignals are more immune to
environmental noises.
1
Surface acoustic wave resonators (SAWs),
2729
lamb wave resona-
tors (LWRs),
30
and lm bulk acoustic wave resonators (FBARs)
3133
are three representative resonators used in pressure-sensing applica-
tions. The propagation speed and wavelength of SAWs are the main pa-
rameters affecting sensor frequency variations.
34
When pressure is
applied to the surface of a sensor, the SAW propagation speed changes
correspondingly. This pressurefrequency relationship forms the sens-
ing mechanism of a typical SAW pressure sensor.
35
The sensing mecha-
nism of LWR and FBAR pressure sensors is determined by pressure-
induced deformations and elasticity variations, which affect either the
dimensions of the resonance cavity or the propagation velocity and
lead to resonant frequency variations.
33,36
Miniaturization of resonant
sensor interface circuits has recently become a research hotspot. In
2015, Nagaraju et al.
32
proposed an extremely miniaturized low-
power sensor interface IC for FBAR pressure sensors (Fig. 1a). Here, a
hermetically sealed reference FBAR was used to eliminate temperature
drifts, and a resolution of 0.037psi was measured. In 2017, Zhang et al.
33
proposed a high-performance FBAR pressure sensor in which the sensor
chip was packaged into an oscillator circuit (Fig. 1b). The sensitivity and
linearity of this sensor were improved by using a partially etched sup-
port lm conguration, and a sensitivity of 0.69 ppm hPa
1
, which is
19% higher than previous results, was obtained.
3. Recent advances in micro/nanoscale pressure sensors
3.1. Novel materials based pressure sensors
3.1.1. 2D materials
Since the discovery of graphene in 2004,
37
2D nanomaterials have
attracted wide research interest due to their unique 2D nature-based
physical and chemical properties. Graphene pressure sensors, which
take advantage of the electrical, mechanical, and piezo-electrical prop-
erties of the bulk material, are of particular interest in this eld.
38
The
Young's modulus of graphene lm is approximately 1 TPa.
39
The elec-
tronic band structure and conduction properties of graphene vary
strongly with the applied pressure, and this principle constitutes the
sensing mechanism of a graphene-based piezoresistive pressure
sensor.
40
Over the last decade, a variety of these sensors with different
design strategies have been developed, and promising results have
been obtained.
4042
Attention has recently been focused on graphene-
based nanocomposites, such as graphene/polyurethane
nanocomposites,
43
graphene/nanowires,
44,45
and graphene/carbon
nanotubes (CNTs), in efforts to improve the sensing performance of
these sensors.
46
Researchers have found that the synergistic effect be-
tween graphene and nanomaterials results in a network with high con-
ductivity and, thus, enhanced sensitivity.
45
Furthermore, inherently
exible graphene-based nanocomposites are ideal materials for E-
skins and other wearable devices. Graphene paper,
47
porous graphene
sponges,
48
and graphene/PDMS sponges
49
have been proven to be
promising materials for exible pressure sensors (Fig. 2).
MXenes are an emerging family of 2D materials with potential appli-
cations in pressure sensing. These 2D materials were rst synthesized
by Naguib et al.
50
and have the chemical formula M
n+1
X
n
T
x
,whereMis
a transition metal, X is C and/or N, and T is a surface functional group.
51
44 Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
MXenes exhibit excellent characteristics, such as high electrical conduc-
tivity, large specic surface areas, and good hydrophilicity,
51
and have
been used for energy storage,
52
catalysis,
53
and water desalination.
54
The wide layer distance of multilayered MXenes enables easy control by
an external pressure, thus indicating that MXenes may also be a promis-
ing material for piezoresistive pressure sensors. In 2017, Ma et al.
55
rst
reported a exible piezoresistive pressure sensor based on multilayered
Ti
3
C
2
MXene with interdigital electrodes (Fig. 3). This sensor showed
high sensitivity below 5 kPa and relatively low sensitivity above 5 kPa,
which is due to the compression limit of MXene layers. This achievement
was followed by a series of reports on piezoresistive pressure sensors
using MXene-based materials, such as MXene/rGO aerogels,
56
porous
MXene-sponge networks,
57
MXenetextile networks,
58
MXene
nanosheets,
59
and MXene/polymer composites.
60
These devices provide
low detection limits, fast response times, and good reproducibility and,
hence, show advantages in the real-time monitoring of weak pressure sig-
nals, such as subtle human activities.
3.1.2. Carbon nanotubes
Since their discovery in 1991, CNTs have attracted considerable in-
terest due to their outstanding mechanical and electrical properties.
61
CNTs have high elasticity and can be bent to very large angles without
breakage.
62
The Young's modulus of single-walled carbon nanotubes
(SWNTs) was estimated to be approximately 1 TPa.
63
CNTs have been
proven to be potential materials for pressure-sensing applications in
numerous studies.
6466
Over the last few years, advances in exible
electronics have produced a new type of CNT/PDMS composite
material-based pressure sensors that can work as articial E-skins to
monitor human physiological signals.
67
Such devices, including capaci-
tive sensors and resistive sensors, exhibit ultrahigh sensitivity to
human motions and good stability under most operating
conditions.
6769
Flexible arrays capable of covering complex surfaces have emerged
as a novel development in CNT-based pressure sensors. In 2017, Zhan
et al.
70
proposed a 4 × 4 array of piezoresistive pressure sensors using
Fig. 1. Miniaturization of resonantpressure sensor interface circuits. (a) Micrograph of a miniaturized sensor interface IC for the FBAR pressure sensorand calibration curveof the sensor.
The interface IC is fabricated by using a 130 nm CMOS process. The maximum error is ±0.53 psi.
32
(b) Photograph of a Colpitts oscillator circuit packaged with an FBAR chip and the
schematic and sensing performance (linear relationship) of FBARs using and not using the partially etched support lm conguration. The partially etched support layer concentrates
the induced pressure in the resonator area, leading to high sensitivity.
33
Fig. 2. Graphene-based exible pressure sensors. (a) Photograph of a graphene paper pressure sensor and itsresponses at different pressures. The sensor shows stable responses at each
tested pressure, and these responsesincrease appreciably over a small pressure range.
47
(b) Schematicand sensing performance of a porous graphene sponge pressure sensor. The sensor
is fabricated by usinga sandwich structure packagedby a PDMS layer, 3Dporous graphene sponges, an interdigitalelectrode, andPET lm. The sensor showsgood stabilityafter 500 cycles
of loading/unloading under 50% strain.
48
(c) Photograph of a graphene/PDMS sponge pressure sensor and its responses at different pressures. The sensoris fabricated by folding a exible
substrate with copper electrodes and using a graphene/PDMS sponge as the dielectric layer. The sensor shows stable responses over seven pressurerelaxation cycles under each test
pressure.
49
45Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
an SWNT/tissue paper composite (Fig. 4a). The sensing array was able to
simultaneously monitor the pressure and position of human physiolog-
ical signals with high sensitivity, low energy consumption, and fast re-
sponse times. In another work, Nela et al.
71
demonstrated a sensing
array of 16 × 16 CNT thin-lm transistors (TFTs) working as E-skins
(Fig. 4b). The response time of this device was much faster than that
of human skin (b30 ms), and the sensing accuracy was not compro-
mised on both at and curved surfaces. Novel sensing structures with
excellent features, including CNT network-covered pyramidal
microstructures,
72
CNT microwires,
73
and wrinkled CNT lms,
74
have
also been demonstrated in pressure sensors.
3.1.3. Metal nanowires
Given their outstanding electrical, optical, and physical properties,
metal nanowires have attracted attention as elements of exible
Fig. 6. PPy-coated paperbased piezoresistive pressuresensor. (a) Schematic of the sensor.
The zigzag layout is inspired by theconcept of leaves uttering in the wind. The inherent
exibility of the paper and conducting polymers allowsthe PPy-coated paper to serveas a
exible sensor possessing good bendability and stability.
88
(b) Response of the sensor to
pressure. The CPFP sensor can easily map pressures with only 1 Pa difference, and the
response time is as low as 100 ms.
88
Fig. 5. Multifunctional sensor array using metal nanowires. (a) Schematic and sensing
performance of an E-skin sensor capable of simultaneously m onitoring pressure and
strain.When pressure is appliedon a sensing pixel, the thickness of the dielectric layer de-
creases,which induces an increasein capacitance. Whenthe sensor is stretched,the plane
strain component parallel to the pre-cracked bers results in an increase in crack density,
which causesa linear increase in resistance.
84
(b) Schematic and sensing performance of a
ngerprint sensor array capableof simultaneously monitoring pressure and temperature.
All transparent sensors for the ngerprint, pressure, and temperature are located in the
central transparent region inside outer bezel areas to interconnect these sensors to the
readout circuit using Cr/Au electrodes. When a nger touches the device, an additional
voltagedrop of approximately 500 mV is generated in theridge area (blue line)compared
with that in the valley area (red line). FETs monitor the tactile pressure (green line), and
the temperature sensor detects the temperature of the nger skin each time the nger
makes contact with it (purple line).
85
Fig. 4. CNT-based exible pressure sensor arrays. (a) Schematic and sensing performance
of a 4 × 4 array of piezoresistive pressure sensors using an SWNT/tissue paper composite.
The composite is assembledonto Au interdigital electrodes on a polyimide (PI) layer, and
PDMS layers are used to seal the sensor and provide mechanical support. The pressure
sensor could be mounted on a human wrist for heart pulse sensing.
70
(b) Schematic and
pressure mapping of a 16 × 16 array of CNT TFTs fabricated into an E-skin. CNT TFTs are
fabricated on a exible PI lm and laminated on a Si handling wafer using PDMS and
epoxy.
71
Fig. 3. An MXene-based piezoresistive pressure sensor. (a) Working principle of the
sensor. The distances between MXene interlayers decrease under an applied pressure,
and the internal r esistance R
C
is reduced. The wi de distance (D
w
) between two
interlayers can easily be compressed, whereas the narrower distance (D
n
) between two
lattices cannot . As a result, th e partial resistivity R
1
of the MXene device is nearly
unchanged under pressure.
55
(b) ITcurves of the senso r at different press ures. The
sensor response rst increases signicantly as a function of pressures below 5 kPa and
then slightly increases at pres sures above 5 kPa due to the compression limit of
narrower distances between two lattices.
55
46 Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
conductors, transparent lm heaters, andphotovoltaic systems.
75,76
The
excellent mechanical properties
77
of metal nanowires also make them
an ideal material for strain and pressure sensors,especially those requir-
ing exibility. In 2014, Gong et al.
78
demonstrated an ultrathin gold
nanowire (AuNW)-based exible pressure sensor. Here, the AuNWs
were deposited onto tissue papers, which were then sandwiched be-
tween a PDMS layer and an interdigitated electrode array-patterned
PDMS layer. External pressures facilitate contact between the AuNWs
and electrodes, resulting in an increase in current. The methodprovided
a low-cost way to develop wearable pressure-sensing devices with rel-
atively high performance (detection limit, 13 Pa) and proved the poten-
tial use of metal nanowires for pressure sensing. Flexible pressure
sensors using silver nanowires (AgNWs)
7982
and copper nanowires
83
have also attracted research interest.
Several reports on multifunctional sensor arrays using metal nano-
wires have been published. In 2017, Cheng et al.
84
reported an E-skin
sensor capable of simultaneously monitoring multiple parameters, in-
cluding pressure and strain. This sensor was based on an elastic AgNW
composite ber electrode and could independently be operated in ca-
pacitive mode for pressure detection and resistive mode for strain de-
tection (Fig. 5a). In 2018, An et al.
85
developed a ngerprint sensor
array integrated with AgNW composite-based pressure-sensitive FETs
and polymer-based temperature-sensitive resistors (Fig. 5b). These
two devices enabled the multifunctional detection of different stimuli
and, thus,greatly expanded theapplication elds of this type of sensors.
3.1.4. Other novel materials
Besides the materials described above, conducting polymer (CP) and
metalorganic framework (MOF)-derived nanostructured materials
have also been studied as potential materials for pressure-sensing appli-
cations. CPs are mainly used as the active layer in piezoresistive pres-
sure sensors for wearable electronics. Conventional piezoresistive
sensors using composites of insulating polymers (e.g., PDMS) and con-
ductive additives are limited by their bulk mechanical properties and,
consequently, offer poor sensitivity and slow response times.
86
By con-
trast, piezoresistive sensors using CPs, especially polypyrrole (PPy), pro-
vide high sensitivity and fast response times due to the conductive and
elastic properties of the active layer.
87
In 2018, Zang et al.
88
reported a
piezoresistive pressure sensor based on PPy-coated paper (Fig. 6). The
device showed a detection limit of 0.3 Pa and a response time of approx-
imately 100 ms and provided a facile and low-cost method to fabricate
high-performance pressure sensors.
MOFs are a family of crystalline nanoporous materials with large
surface areas and high porosity. These materials have received world-
wide attention for their potential applications in gas separation, chemi-
cal sensing, and heterogeneous catalysis.
89
In particular, MOFs can be
used as precursors/templates to prepare nanostructured materials
with large pore volumes and surface areas and excellent electrical sta-
bility. Fu et al.
90
rst proposed a resistive pressure sensor based on
MOF-derived nanowire arrays, the sensing mechanism of which was at-
tributed to mechanical contact between two opposite nanowire arrays
(Fig. 7). In another work, Zhao et al.
91
demonstrated a multifunctional
sensor using MOF-derived porous carbon to have high performance in
pressure and temperature sensing due to its porous structures and
rough surface.
3.2. Flexible pressure sensors
Due to rapid developments in electronic sensing technologies and
organic electronic technologies, wearable sensors have been widely de-
veloped over the last several decades.
9294
Flexible pressure sensors are
of particular interest and importance in wearable electronics owing to
their broad application prospects in human-machine interfaces,
95,96
E-
skins,
97,98
robotics,
99,100
and health care systems.
13,101
An ideal exible
pressure sensor should have the advantages of high sensitivity, fast
Fig. 8. Schematic and sensing performance of a exible resistive pressure sensor based on printing paper patterned with Ag interdigital electrodes. The exible and sensitive resistive
pressure sensor is composed of carbonized crepe paper (CCP) as the active material and printing paper as the substrate. The conductive CCP and interdigitated electrodes on printing
paper are combined and encapsulated by PI tape to prepare the pressure sensor. Weights of 1 and 2 g lying on the sensor array are immediately illustrated by the pixel bars using
different heights in the 3D bar graph.
103
Fig. 7. Schematic and sensing performance of a resistive pressure sensor based on MOF-
derived nanowire arrays. The conducting path of the sensor is constructed by numerous
mechanical contacts between the nanowire arrays . A metal coin is left on the sensor
arrays, and the pressure distributions are revealed through current mapping of these
arrays.The sensor arrays are ableto give spatially resolved pressure change information.
90
47Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
responses, strong robustness, low cost, and long lifetime to meet the de-
mands of these emerging technologies. Indeed, establishing compatibil-
ity between exible pressure sensors and the array upon integration has
become a major challenge for further development because large-area
measurements, which can provide comprehensive information about
the test object, are also needed in these applications.
Flexible pressure sensors are composed of three key parts: sensing
materials, electrodes, and substrates.
14
Advances in exible sensing ma-
terials (e.g., graphene, MXenes or nanocomposites) and electrodes were
discussed in the aforementioned sections. Rubbery polymers for exible
substrates, including PDMS, polyethylene terephthalate (PET), and PI,
are widely discussed in the literature because of their excellent exibil-
ity, stability, and mechanical properties.
13
In fact, paper substrates have
attracted considerable research attention because of their unique prop-
erties of low cost andeasy realization.
102
For instance, Chenet al.
103
pro-
posed a exible resistive pressure sensor based nearly completely on
paper in 2017 (Fig. 8). The substrate of the sensor was printing paper
patterned with Ag interdigital electrodes via the screen-printing tech-
nique, and the sensing material was formed by using carbonized crepe
paper. Experimental results showed that the paper-based sensor offers
excellent performance (detection limit, 0.9 Pa; response time, b30 ms)
and good durability (over 3000 cycles). Gao et al.
104
reported a paper-
based resistive pressure sensor that could be used as an E-skin to
monitorapplied pressure signals. Despite these achievements, however,
the high hygroscopicity and weak mechanical strength of paper-based
materials pose considerable challenges that must be overcome for prac-
tical applications.
Self-healing is a new trend in exible pressure-sensing devices.
Given the advantages presented by the specic association/dissociation
of molecular bonds, self-healing materials, especially E-skins, are able to
repeatably heal damage and recover mechanical and electrical proper-
ties to extend the service life of sensing devices.
97
In 2018, Zou
et al.
105
proposed a covalent thermoset nanocomposite-based
rehealable E-skin capable of monitoring pressure, temperature, ow,
and humidity (Fig. 9). The self-healing material of this device was com-
posed of dynamic covalent thermoset polyimine-doped Ag nanoparti-
cles. The self-healing process of the E-skin was achieved by new
oligomers/polymers growing across the damage site to mimic the
healing process of injured skin.Moreover, the mechanical and electrical
properties of the device could be restored after the self-healing process.
3.3. Implantable pressure sensors
Implantable pressure sensors that are small in size, light in weight,
and compatible with body tissues are extremely necessary to realize
the real-time monitoring of physiological parameters in the human
body for clinical medicine. Research on implantable pressure sensors
has extended to various aspects of health, including blood pressure
(monitoring of hypertension and heart failure),
106108
intraocular pres-
sure (detection of glaucoma),
109111
intracranial pressure (monitoring
of intracranial hypertension),
112114
and bladder pressure (detection
of urinary incontinence).
115
However, several challenges in designing
and developing implantable devices for in vivo pressure measurements
remain; these challenges include packaging of devices, long-term accu-
racy of signals, biocompatibility of materials, wireless transmission of
data, and external power.
MEMS sensors based on micromachining technology provide new
opportunities for developing miniaturized and low energy-consuming
implantable pressure-sensing devices. MEMS sensors can leverage ad-
vances in biocompatible packaging
116
and wireless data and power
transmission,
117
leading to improvements in conventional implantable
pressure sensors. Capacitive and piezoresistive sensors using deform-
able membrane structures are the two main types of MEMS-based im-
plantable pressure sensors. For instance, Chen et al.
118
demonstrated a
capacitive implantable pressure sensor using a goldPI diaphragm con-
guration in 2017 (Fig. 10). Here, a medical-grade stainless steel sub-
strate was utilized to ensure the complete biocompatibility of the
device. The capacitive structure comprised an air-lled cavity
microfabricated on the substrate and a gold-PI diaphragm that seals
Fig. 10. (a)Cross-sectionalstructure and (b)fabrication process of an implantable pressure sensorbased on a goldPI diaphragm conguration.The sensor comprisesa stainless-steel(SS)
chip micromachined to have a square cavity serving as one of the capacitive electrodes and a goldPI multilayer diaphragm that hermetically seals the cavity while acting as another
capacitive electrode to deect external pressure. The capacitive structure is constructed by heat-assisted bonding of the PI side of the diaphragm to the SS chip.
118
Fig. 9. Schematic of the rehealability and recyclability of an E-skin. When moderately
damaged, the E-skin can be rehealed. The rehealed E-skin can restore mechanical and
electrical properties to levels comparable with those of the original device. When severe
damage occurs or the device is no longer needed, the whole E-skin can be completely
recycled, leaving no waste. Once recycled, a short-oligomer/precursor solution and Ag
nanoparticles that can be used to make new materials and devices are obtained.
105
48 Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
the cavity and serves as the capacitive electrode. Deection of the dia-
phragm by an applied pressure resulted in capacitance variations be-
tween the Au side of the diaphragm and the substrate. Unfortunately,
these membrane-based sensors usually offer high sensitivity and
biostability but suffer from long-term stability issues due to material fa-
tigue of the membrane substrate.
119
Thus, this issue must be further
overcome in the next stages of development.
3.4. Self-powered pressure sensors
Harvesting energy directly from the environment may ef-
ciently solve the threat of global energy exhaustion.
120,121
Hence,
self-powered pressure sensors have been extensively studied in
recent years. Since mechanical energy is an easily available energy
resource in daily life, the use of the triboelectric effect, which con-
verts mechanical energy into electricity, is of vital importance for
a self-powered device. Triboelectric generators are the most
widely used devices for producing energy in self-powered sys-
tems. For instance, Fan et al.
122
and Yang et al.
123
proposed two
types of triboelectric nanogenerator (TENG) devices based on a
micropatterned plastic lm substrate and paper substrate, respec-
tively, for use as self-powered pressure sensors. The self-powering
mechanism of TENG is based on the collection of mechanical en-
ergy from human motion (Fig. 11). When pressure is applied to
a TENG, the deformation of the device leads to a change in electric
outputs. Following these works, several researchers have
attempted to improve the performance of triboelectric effect-
based devices using various materials, including graphene
oxide,
124
polymer sponges,
125
and nanobers.
126,127
These devices
present the advantage of simple and low-cost preparation and
show potential for scaling up for large-scale production.
Self-powered sensing arrays based on triboelectric effects have
been proposed. In 2013, Lin et al.
128
rst proposed a 6 × 6 array of
triboelectric active sensors for pressure detection. Here, each sensor
consisted of a PDMS membrane with pyramidal microstructures,
and an Al lm assembled with Ag nanowire/nanoparticle composite
was applied to improve the triboelectric effect. Spatial pressure
mapping could be achieved by integrating multiple sensors into a
sensing array. Self-powered pressure sensor arrays based on the tri-
boelectric effect have been proposed to meet the demands of practi-
cal applications. In 2017, for example, Ma et al.
129
reported a self-
powered E-skin consisting of a network of triboelectric pressure
sensors using PDMS layers and carbon ber electrodes (Fig. 12).
This device could be assembled on a nger or beetle for pressure
monitoring with an ultra-high resolution of 127 × 127 dpi. In the
same year, Yuan et al.
130
proposed a self-powered exible triboelec-
tric sensing array for touch-screen applications. This sensing array
was constructed using lms of PDMS, uoroethylene
uoropropylene copolymer, and a PET substrate sandwiched be-
tween two ITO electrodes. The sensing array was capable of sensing
real-time touch, mapping spatial pressure distributions, and track-
ing touch movements.
Fig. 12.Schematic of the structure and performance of a self-powered E-skinconsistingof triboelectric pressure sensorsusing PDMS layersand carbon ber electrodes. The top insetshows
an enlarged diagram of one pixel, the bottom-left insetshows an SEM image of a single carbonber, and the bottom-rightinset shows a micrograph of onepixel. A tip is controlled by a
linear motor to press the pixels of the device with variable forces. Real-time mapping of the pressure trajectory could be easily achieved.
129
Fig. 11. Working mechanism of a TENGdevice. The operating principle of TENGis based on
the periodic contact and separation of two materials with contrast ing triboelectric
polarities. Contact between the se materials pro duces triboelectric charged surfaces.
During contact and separation, potential differences are created and contrib ute to the
ow of electronsbetween the back conductive electrodes to generate electric outputs.
123
49Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
4. Conclusions
Micro/nanoscale pressure sensors have been extensively developed
and studied over the years due to their increased miniaturization and
performance. In this review, the sensing principles of current
pressure-sensing devices were summarized, and recent advances in
the development of micro/nanoscale pressure sensors with respect to
emerging markets, including novel material-based, exible, implant-
able, and self-powered pressure sensors, were discussed.
Although progress has been made in these areas, further work and
research should be conducted to tackle the remaining challenges in
practical applications and commercial exploitation. Considering the sce-
narios associated with smart wearable devices and health monitoring
systems, the development trends of micro/nanoscale pressure sensors
may focus on the following issues. First, while various pressure-
sensitive materials have been investigated for implementation in
micro/nanoscale pressure sensors, realization of an active material for
repeatable and uniform mass-production remains a challenge. Second,
construction of a versatile pressure sensor array with small pixel sizes
and large coverage areas is necessary for sensor network-related appli-
cations (e.g.,E-skins). The current approach integrates individual sen-
sors capable of monitoring other factors, such as temperature,
humidity, and ow.
105
Therefore, crosstalk between sensors and inter-
actions between environmental factors should be considered in the de-
sign of these materials and sensors. Third, further development of
highly sensitive pressure sensors for health monitoring is necessary.
Current implantable and self-powered pressure sensors provide poten-
tial solutions for future in vivo applications. However, more research
work should be dedicated to the realization of high sensing perfor-
mance, miniaturized circuit components, and effective wireless trans-
mission. Overall, considering the rapid development and advancement
of micro/nanoscale pressure sensors, commercialization of these de-
vices and their use in wider applications may be expected in the near
future.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inu-
ence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (NSFC Nos. 61674114, 91743110, 21861132001), National Key
Research and Development Program of China (No. 2017YFF0204604),
Tianjin Applied Basic Research and Advanced Technology (No.
17JCJQJC43600), the Foundation for Talent Scientists of Nanchang Insti-
tute for Microtechnology of Tianjin University, and the 111 Project (No.
B07014).
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Ye Chang received the B.S. degree from Tianjin University,
Tianjin, China, in 2013, where he is currently pursuing the
Ph.D. degree. His research interestsinclude lm bulkacoustic
resonator chemicalsensors and chemicalfunctionalization of
micro/nanodevices.
Jingjing Zuo received her B.S. degree in biomedical engi-
neering from Dalian University of Technology, Dalian,
China, in 2017. She is currently pursuing her M.S. degree in
Tianjin University. Her research interests focus on pressure
detection based on MEMS resonators.
Hainan Zhang received her Ph.D. d egree at University of
Twente, Enschede, Netherlands (2016). Currently, she is an
assistant professor at State KeyLaboratory of Precision Mea-
suring Technology & Instruments, Department of Precision
Instrument Engineering of Tianjin University. Her research
interests focus mainly on MEMS devices and microuidics.
Xuexin Duan received his Ph.D. de gree at University of
Twente, Netherland (2010). Af ter Postdoc stud ies at Yale
University, he moved to Tianjin University. Currently, he is
a full professor at State Key Laboratory of Precision Measur-
ing Technology & Instruments, Department of Precision In-
strument Engineering of Tianjin University. His research is
about MEMS/NEMS devices, microsystem, microuidics and
their interfaces withchemistry, biology, medicine, and envi-
ronmental science.
52 Y. Chang et al. / Nanotechnology and Precision Engineering 3 (2020) 4352
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Compressible and conductive materials (CCMs) have important applications in developing pressure sensors for various wearable devices. However, it is a great challenge to fabricate a CCM with superior mechanical properties and ultrahigh linear sensitivity. Herein, we developed a green and effective method to fabricate a lightweight, compressible and conductive aerogel by connecting Ti 3 C 2 nanosheets into continuous and ordered lamellae with a biomass polymer (chitosan). Due to the connecting effect, the lamellae are flexible, highly compressible and elastic as well as structurally stable. These features allow the aerogel to withstand extremely high strain (99%), long-term compression (up to 150000 cycles), and repeated bending. Furthermore, due to the unique lamellar architecture, the aerogel demonstrates an ultrahigh sensitivity (80.4 kPa ⁻¹ ) and exceptionally wide linear range (within strain of 0.5-70%). In addition, it has low detection limits for tiny strain (0.5%) and pressure (1.0 Pa). Due to these advantages, the aerogel shows potential for application in flexible wearable devices for detecting biosignals.
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Wearable sensors with excellent flexibility and sensitivity have emerged as a promising field for health care, e‐skins, etc. Three‐dimensional (3D) graphene sponges (GS) have emerged as high performance piezoresistive sensors, however, problems such as limited flexibility, high‐cost and low sensitivity remained. Meanwhile, device‐level wearable pressure sensor with GS was rarely demonstrated. In this work, a highly ordered 3D porous graphene sponges (OPGSs) was successfully prepared and controlled by emulsion method at first, and then a device‐level wearable pressure sensor with high flexibility and sensitivity was assembled with Au‐electrode and polydimethylsiloxane (PDMS) for the reliable package. The pH values were carefully controlled to get a stable emulsion and the OPGS showed highly ordered 3D structure with ultralow density, high porosity and conductivity which proved a gauge factor of 0.79‐1.46 with 50% compression strain and excellent long‐term reproducibility in 500 cycles of compressing‐relaxing. Moreover, the well‐packaged pressure sensor devices exhibited ultra‐high sensitivity to detect human motions such as human wrist bending, elbow bending, finger bending and human palm. Thus, the developed pressure sensors here exhibited great potential in fields of human‐interactive applications, biomechanical systems, e‐skin, etc.
Article
Flexible and degradable pressure sensors have received tremendous attention for potential use in transient electronic skins, flexible displays, and intelligent robotics due to their portability, real-time sensing performance, flexibility, and decreased electronic waste and environmental impact. However, it remains a critical challenge to simultaneously achieve a high sensitivity, broad sensing range (up to 30 kPa), fast response, long-term durability, and robust environmental degradability to achieve full-scale biomonitoring and decreased electronic waste. MXenes, which are two-dimensional layered structures with a large specific surface area and high conductivity, are widely employed in electrochemical energy devices. Here, we present a highly sensitive, flexible and degradable pressure sensor fabricated by sandwiching porous MXene-impregnated tissue paper between a biodegradable polylactic acid (PLA) thin sheet and an interdigitated electrode-coated PLA thin sheet. The flexible pressure sensor exhibits high sensitivity with a low detection limit (10.2 Pa), broad range (up to 30 kPa), fast response (11 ms), low power consumption (10-8 W), great reproducibility over 10000 cycles, and excellent degradability. It can also be used to predict the potential health status of patients and act as an electronic skin (E-skin) for mapping tactile stimuli, suggesting potential in personal healthcare monitoring, clinical diagnosis, and next-generation artificial skins.
Article
Recently, cellulose paper based materials are emerging for applications in wearable “green” electronics due to their earth-abundance, low cost, light weight, flexibility, and sustainability. Herein, for the first time, we develop an almost all cellulose paper based pressure sensor through a facile, cost-effective, scalable, and environment-friendly approach. The screen-printed interdigital electrodes on the flat printing paper and the carbonized crepe paper (CCP) with a good conductivity are integrated into a flexible pressure sensor as substrates and active materials, respectively. The porous and corrugated structure of the CCP endows the pressure sensor with high sensitivity (2.56-5.67 KPa⁻¹ in the range of 0-2.53 KPa), wide workable pressure range (0-20 KPa), fast response time (<30 ms), low detection limit (~0.9 Pa), and good durability (>3000 cycles). Additionally, we demonstrate the practical applications of the CCP pressure sensor in detection of finger touching, wrist pulse, respiration, phonation, and acoustic vibration, etc., and real-time monitoring of spatial pressure distribution. The proposed CCP pressure sensor has great potentials in various applications as wearable electronics. Moreover, the subtle fabrication of desired materials based on commercially available products provides new insights into the development of green electronics.